Added "Invokable Cases" PHPStan extension (#13)

* Added "Invokable Cases" PHPStan extension

This includes a PHPStan extension to add support for `InvokableCases`
so that static analysis tools can understand the callable methods and
their return types.

The extension has been added in a way that will allow multiple extensions
in the future if required, with a single include file that will import
all extensions, or the option to include only specific extensions.

* Added "Invokable Cases" PHPStan extension testing

tests/PHPStan/InvokableCases/Suits.php

@@ -0,0 +1,30 @@
+<?php
+
+declare(strict_types=1);
+
+namespace ArchTech\Enums\Tests\PHPStan\InvokableCases;
+
+use ArchTech\Enums\InvokableCases;
+
+enum Suits: string
+{
+    use InvokableCases;
+
+    case clubs = 'C';
+    case diamonds = 'D';
+    case hearts = 'H';
+    case spades = 'S';
+
+    public static function valuable(): self
+    {
+        return self::diamonds;
+    }
+
+    public function isRed(): bool
+    {
+        return match ($this) {
+            self::diamonds, self::hearts => true,
+            default => false,
+        };
+    }
+}